In a LTE HetNet (heterogeneous network), there are different kinds of low-power cells including pico cells, femto cells and relay cells, besides the traditional macro cells. Due to the large disparity between the transmit power level of macro eNBs and small eNBs, the downlink range of a small eNB is much smaller than that of a macro eNB. If cell selection is predominanty based on the downlink signal strength, the usefulness of small eNB will be greatly diminised.
The larger range of high-power eNBs limits the advantage of cell splitting by attracting most user equipments to macro eNBs based on signal strength without having enough macro eNB resources to efficiently serve these use equipment; meanwhile, low-power eNBs may not be serving any user equipments. Even if all the low-power eNBs can use available spectrum to serve at least one user equipment, the difference between the loadings of different eNBs can result in an unfair distribution of data rates and uneven user experiences among the user equipments in the network.
Therefore, from the perspective of network capacity, it is desirable to balance the load between macro and small eNBs by expanding the coverage of small eNBs and subsequently increase cell splitting gains. As shown in FIG. 1, wherein the range of pico cells is expanded, this concept is referred to as CRE (cell range expansion) which is an effective way for load balancing in HetNets.
The CRE may be realized by adjusting cell reselection parameters and handover parameters for idle UEs and connected UEs respectively. For the idle UEs, by adjusting the cell reselection parameter Qoffset at the macro cell, it may reselect a neighboring small cell; by adjusting Qhyst at the small cell, it remains residing in the small cell. Correspondingly, for the connected UEs, by increasing CIO (CelllndividualOffset) of a neighboring small cell, it may handover from the macro cell to the small cell early; by reducing CIO of a neighboring macro cell, it may handover out of the small cell later. It should be noted that adjusting CIO of one neighboring cell does not impact handover metrics towards other noninvolved cells.
Obviously, in the connected mode, CRE shifting the cell border virtually is a cell pair concept and so should be coordinated on cell pair basis. Once the macro user is handed over for small cell region expansion, it may be necessary to amend the handover parameters such as CIO, so that the user will not be handed over back to the macro cell due to signal reasons. The amendment must be performed in both cells, so that CIOs between the two cells remain coherent. In other words, in the case of load balancing, the change of handover parameters such as CIO is usually symmetrical: if the macro cell increases the CIO of its neighboring small cell, for example, by 2 dB, it means the cell border of the macro cell is moved −2 dB; meanwhile, the small cell should decrease the CIO of its neighboring macro cell by 2 dB to expand its own cell border by +2 dB. Here, the 2 dB is known as CRE biasing towards the small cell.
However, according to the current definition in LTE-related specifications, e.g., the definition in Release 11 published in June 2012, i.e., TS 36.331 v11.0.0 “Radio Resource Control (RRC); Protocol specification (Release 11), the handover parameters such as CIO are applied for all user equipments.
Handover Parameter Setting
In the connected mode, CRE biasing is applied for a UE in relation to handover to a new target cell. This is performed by measurement event A3. When the RSRP (Reference Signal Receiving Power) or RSRQ (Reference Signal Receiving Quality) of a neighboring cell is a given HOM (Handover Margin) better than the current serving cell for at least the duration of TTT (Time-to-Trigger), a handover decision is made to the UE to hand it over from the serving cell to the target cell.
An exemplary A3 trigger condition may be defined as:Mn+Ofn+Ocn−Hys>Mp+Ofp+Ocp+Off  (1)
where Mp and Mn are the RSRP or RSRQ levels of the serving cell and the target cell. Ofp and Ofn are the frequency specific offsets of the serving cell and the target cell. Ocp and Ocn are the cell specific offsets of the serving cell and the target cell, i.e., CIO. Hys is the hysteresis parameter common for all measurement events; Off is the offset parameter for event A3 regardless of neighboring cells.
The condition (1) may be simplified as below:Mn>Mp+HOM(p,n)  (2)
where HOM(n, p)=Ofp+Ocp+Off−Ofn−Ocn+Hys is usually called as the handover margin (HOM). That HOM(p, n) is a positive value indicates the handover is proposed to take place later. Obviously, HOM is a cell pair specific offset which shifts the cell border virtually. And as stated above, HOM should be coordinated on cell pair basis.
These handover parameters are either cell-specific or event-specific, but technically eNB could signal UE specific values to different UEs via dedicated RRC signalings. And then, the handover should be performed at the same point, including from macro cell to pico cell and from pico cell to macro cell, i.e., the neighboring cell should be aware of the individual values. Currently this is not possible via the X2 interface, which typically applies the same handover parameters to the UEs throughout the cell or even the network.
Mobility Settings Change
As above mentioned, once the macro cell user is handed over for range expansion of the small cell, it may be necessary to amend the handover parameter such as CIO, so that the user will not be handed over back to the macro cell due to signal reasons. The amendment must be performed in both cells, so that handover settings between the two cells remain coherent. For this purpose the Mobility Settings Change X2 procedure is defined in the LTE related specifications, e.g., TS 36.423 v11.0.0, “X2 application protocol (X2AP) (Release 11)” published in March 2012. The eNodeB that detects the change needs to estimate how much the cell border would need to be shifted to avoid quick return of the user. The shift of cell border is expressed in dBs and informed to the corresponding other cell in MOBILITY CHANGE REQUEST X2AP message as shown in FIG. 2. In the case of load balancing, the change is usually symmetrical: if the macro cell shifts its handover border by, for example, −2 dB, the small cell should extend its own by +2 dB.
However, only one signal HOM can be changed via the Mobility Settings Change procedure. It may be implemented by including the mobility parameters information IE (Information Element) within the MOBILITY CHANGE REQUEST message.
With reference to FIG. 2, the MOBILITY CHANGE REQUEST message is transmitted from eNB 1 to eNB 2 to initiate adaptation of the mobility parameters. The Table 1 below shows an exemplary MOBILITY CHANGE REQUEST message:
TABLE 1an example of MOBILITY CHANGE REQUEST messageIE type andSemanticsAssignedIE/Group NamePresenceRangereferencedescriptionCriticalityCriticalityMessage TypeM9.2.13YESrejecteNB1 Cell IDMECGIYESreject9.2.14eNB2 Cell IDMECGIYESreject9.2.14eNB1 MobilityOMobilityConfigurationYESignoreParametersParameterschange in eNB1Informationcell.9.2.48eNB2 ProposedMMobilityProposedYESrejectMobilityParametersconfigurationParametersInformationchange in eNB29.2.48cell.CauseM9.2.6YESreject
The Mobility Parameters Information IE contains the change of the Handover Trigger with respect to its current value. The Handover Trigger corresponds to the threshold at which a cell initialises the handover preparation procedure towards a specific neighbour cell. That this change is a positive value indicates the handover is proposed to take place later. Table 2 below shows an exemplary Mobility Parameter Information IE:
TABLE 2an example of Mobility Parameter Information IEIE/GroupIE typeSemanticsNamePresenceRangeand referencedescriptionHandoverMINTEGER (−20 . . . 20)The actualTriggervalue isChangeIE value *0.5 dB.
Based on the above description, under the assumption of a fixed starting point, the information exchange via the Mobility Settings Change procedure could work only for a common set of HOM for each cell to cell relationship. This cannot support UE-specific or UE group-specific HOM exchange, i.e., it cannot set a CRE biasing value corresponding to the UE capability, thereby unable to realize a more flexible CRE load balancing.
The LTE's definition on the handover parameters such as CIO does not consider the receiver capability of UE. Obviously different UEs may have different capabilities to deal with severe interference in CRE region especially on control channels (even when ABS (Almost Blank Subframes) are used for eICIC (enhanced Inter-cell Interference Coordination), system signals such as CRS are still transmitted). For example, UEs supporting Release 8/9/10 may accept at most 6 dB biasing, but UEs supporting Release 11 with interference-cancellation receivers could work under larger biasing such as 9 dB, and UEs supporting Release 12 are expected to work under even larger CRE biasing with advanced receiver together with some network-assist signalings. Additionally, Rel-8/9 UEs cannot use 2 restricted measurement subsets for RRM (Radio Resource Management) and CSI (Channel State Information), so that may not work well in the CRE region even with ABS protection. In another example, besides DL signal strength, CRE may also be implemented based on other metrics such as DL pathloss or network utility. In order to implement DL pathloss or network utility based CRE, handover parameters such as CIO need to be enhanced on UE basis to map the DL signal strength to the associated UE.
For high-speed UEs, in the connected node, biasing may be used to reduce the handover rate, thereby decreasing handover failure. In this case, biasing (negative) is used to prevent the UE from being handed over to a small cell. It further requires a UE-speed specific biasing.
Therefore, the cell specific handover parameter is not flexible enough for CRE load balancing, but advantageous to the UE specific or UE group specific CRE, i.e., performing CRE with differentiation of UE's capability or speed to improve network efficiency and user QoS.
Besides, the prior art also proposes service class dependent load balancing for self organizing networks, which requires to expand single handover parameter setting to support distinct service class and correspondingly enhance the coordination of individual handover parameter setting for each distinct service class between neighboring cells. Obviously, the above service class based load balancing does not include the scenario of CRE. Further, the CRE biasing applied applied to a UE does not depend on its service class but on its attribute information such as receiver capability or mobility speed.